doc/go1.14: document http.ServeFile large file fix for Windows
    
    Document that for Windows, net/http.ServeFile can now
    correctly serve files >2GB after we fixed internal/poll.SendFile
    to transmit files larger than the default Windows limit of 2GB.
    
    References: CL 192518, CL 194218
    
    Updates #36878
    
    Change-Id: Ibefc8b2841bc0cee3a89884a680085f99d2b6928
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217779
    Reviewed-by: Ian Lance Taylor <iant@golang.org>